Method and apparatus for crosslinking individualized cellulose fibers

ABSTRACT

An apparatus is disclosed for preparing a quantity of individual treated fibers from one or more fiber mats. The apparatus comprises a fiber treatment zone, and a conveyor for conveying each mat through the fiber treatment zone. In the treatment zone each mat is impregnated by an applicator with a treatment material, such as a crosslinking substance, and conveyed directly to an attrition device. The attrition device fiberizes the mats to form a fiber output having a low nit level, such as no more than about three, and a dryer both dries the fiber output and cures the crosslinking substance. The fiberizer is configured to minimize the accumulation of fiber at locations therein. Fiber is transported from the attrition device to the dryer at a high velocity under reduced pressure to promote drying. A heated retention bin is provided after drying to increase curing time in the system. A thermobonding agent may be added to the dried and cured fibers to enhance the wet strength of webs made from the fiber.

This application is a continuation of application Ser. No. 07/820,323,filed Jan. 13, 1992 now U.S. Pat. No. 5,437,418.

CROSS-REFERENCE TO RELATED CASES

This is continuation-in-part of pending United States patentapplications Ser. No. 07/665,761, filed Mar. 7, 1991 now U.S. Pat. No.5,252,275; and Ser. No. 07/607,268, filed Oct. 31, 1990 abandoned, whichis a continuation-in-part of Ser. No. 07/395,208, filed Aug. 17, 1989now 5,225,047, which is a continuation-in-part of application Ser. No.07/284,885, filed Dec. 15, 1998 abandoned, which is acontinuation-in-part of application Ser. No. 07/140,922, filed Dec. 28,1987 abandoned, which is a continuation-in-part of application Ser. No.07/004,729, filed Jan. 20, 1987 abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a fiber treatment apparatus and moreparticularly to the apparatus of the type which utilizes sprayers orother applicators to treat a fiber mat and mechanisms for subsequentlyfiberizing the mat following such treatment.

2. General Discussion of the Background

Various devices are known in the art for treating fibers withcrosslinking agents in mat form and thereafter breaking the mats intoindividual fibers. For example, U.S. Pat. No. 3,440,135 to Chungdiscloses a mechanism for applying a crosslinking agent to a cellulosicfiber mat, then passing the mat while still wet and following “aging,”through a fiberizer, such as a hammermill to fiberize the mat, anddrying the resulting loose fibers in a two stage dryer. The first dryerstage is at a temperature sufficient to flash water vapor from thefibers and the second dryer stage is at a temperature that effectscuring of the crosslinking agent. A cyclone separator is thenillustrated separating the fibers from the gas and for subsequentcollection. Chung mentions the need for the “aging” step, of many hoursduration, in order to reduce the level of nits in the resulting fiberproduct. As described below, nits are typically interbonded fibers whichcan interfere with product quality. Therefore, the Chung apparatussuffers from the drawback of requiring the inconvenient and costlystorage of wet fiber mats (e.g. in roll form) for a substantial periodof time in order to minimize nit formation.

Unfortunately, fiberization processes known in the art which employcurrently available fiberizing or comminution machinery yieldcrosslinked fibers that have too many nits and knots to be acceptablefor many uses. A probable reason is that such machinery has excess deadspace where fibers are excessively pressed together and/or has localizedregions of elevated temperature hot enough to cause premature curing ofthe crosslinking agent while fibers are in intimate contact with eachother. Since fiberization is performed on a mat that is still wet withthe uncured crosslinking agent, dead spaces and hot spots in thefiberizer would encourage the formation of interfiber bonds, which formnits, that virtually cannot be broken by downstream equipment.

Interfiber bonding in a conventional fiberizer apparatus can also leadto production of excessive amounts of “fines,” which are undesirablyshort fibers due principally to fiber breakage. Crosslinking impartssubstantial brittleness to cellulose fibers, which thereby exhibitlimited compliance to mechanical stresses. Nits are especiallysusceptible to mechanical stresses because of their density which ismuch greater than the density of individual fibers. Excess fiberbreakage and fines not only degrade absorbency but can substantiallyreduce the loft and resiliency of a product made from crosslinkedfibers.

One approach to reducing fines is to diminish interfiber crosslinking,as in published European Patent Application Nos. 427,316 A2; 429,112 A2;427,317 A2; and 440,472 A1, as well as in copending United States patentapplication Ser. No. 07/607,268, filed Oct. 31, 1990. A drawback to thisapproach is that the substantial elimination of interfiber bondsproduces a web having low tensile strength. Wet laid sheets made fromsuch fibers tend to fall apart, and are unsuitable for many industrialapplications.

Yet another problem with prior processes is that output from the systemis so rapid that fibers treated with the crosslinking agent do not havesufficient time to cure after they are fiberized and dried. Curing timecan be increased by lengthening the conduit through which fiberized anddried material passes, but such a solution is expensive. Lengtheningconduits requires a large capital investment that reduces costefficiency.

Hence, there is a need for an apparatus that will produce treatedfibers, such as intrafiber crosslinked cellulose, having a nit levellower than levels obtainable with existing equipment. There is also aneed for such an apparatus that will produce fibers from a mat comprisedof crosslinked cellulose while not causing significant breakage ofindividual fibers of the mat.

It is an object of this invention to provide such an individualized,intrafiber crosslinked cellulose web that has improved wet tensilestrength.

It is another object of the invention to provide a process for producingan individualized, intrafiber crosslinked product that providesincreased curing time for crosslinking to progress after the fibers aredried.

It is yet another object of the invention to provide such a process thathas improved flash drying of moisture from the fibers prior to curing.

It is another object of the invention to enhance the uniformity ofcrosslinking agent application to a fibrous mat.

Another object of the present invention is to provide an apparatus andmethod for producing treated fibers, such as crosslinked cellulosefibers, with a low nit level and preferably a nit level no greater thanabout three.

Another object is to provide such an apparatus and method thatcomminutes one or more mats of non-crosslinked cellulose fibers whichhave been impregnated with a crosslinking substance, where thecomminution is performed before the crosslinking substance is dried andcured.

Another object is to provide such an apparatus and method that minimizesthe breakage of individual fibers.

Another object is to provide such an apparatus and method that yieldscrosslinked fibers having substantially no knots.

It is yet another object to provide a crosslinking process that operatesat a pH that is compatible with standard unmodified papermakingequipment.

Finally, it is an object of the present invention to provide a sheethaving high bulk wet resilience and good porosity into which liquidimpregnants can be efficiently introduced.

These and other objects of the invention will be understood more clearlyby reference to the following detailed description and drawings.

SUMMARY OF THE INVENTION

The apparatus of the present invention is particularly adapted forpreparing a quantity of individual crosslinked cellulose fibers from oneor more mats comprised of non-crosslinked cellulose fibers. Theapparatus comprises: an applicator that applies a crosslinking substanceto a mat of cellulose fibers at a fiber treatment zone; a fiberizerhaving a fiberizer inlet; a conveyor that conveys the mat through thefiber treatment zone and directly to the fiberizer inlet withoutstopping for curing. The fiberizer provides sufficient hammering forceto separate the cellulose fibers of the mat into a fiber output ofsubstantially unbroken individual cellulose fibers. A dryer coupled tothe fiberizer receives the fiber output, dries the fiber output, andcures the crosslinking substance, thereby forming dried and curedfibers. The fiberizer preferably fiberizes the treated mat to form afiber output having a low nit level, such as a nit level of no more thanabout 3.

The apparatus also includes a reduced pressure conduit between thefiberizer and dryer in which the individual cellulose fibers are heatedand the velocity of their flow is increased after they leave thefiberizer. This conduit opens into an expansion chamber that allows thefiber flow to expand and increase fiber separation. The flow velocity ofthe fibers in the conduit is preferably increased by reducing thediameter of the conduit between the fiberizer and dryer. A downstreamconnection between the conduit and dryer gradually increases in diameterto provide an expansion zone between the conduit and expansion chamber.The reduced diameter conduit provides an area of reduced pressure thatpromotes evaporation of moisture from the fibers of the conduit. Theexpansion chamber subsequently provides another evaporation zone inwhich moisture is quickly and explosively released from the fibers,thereby further enhancing their separation and production ofindividualized fibers.

The apparatus further includes a hot air blower that blows hot air intothe conduit toward the expansion chamber. Fibers are introduced into theconduit between the blower and expansion chamber through a fiberintroduction inlet. The blower preferably introduces hot air at atemperature of about 260° C. into the conduit to transport the fibersand reduce their moisture content in the reduced pressure environment ofthe necked down conduit.

The apparatus also preferably includes a heated retention chamber intowhich dried, treated fiber output is introduced for a preselected periodof time to allow curing of the crosslinking substance. In someembodiments, the retention chamber is positioned between the flashdrying and curing chambers of the dryer. In other embodiments, theretention chamber is downstream from the curing chamber, for example,after a cyclone separator that separates fibers received from the curingchamber. In especially preferred embodiments, the retention chamber hasthe shape of an inverted pyramid with an open base through which thefibers are introduced. The apex of the pyramidal chamber can selectivelybe opened and closed to control the movement of cured fibers out of thebottom of the retention chamber.

Representative conveyors include, but are not limited to, conveyor beltsand roller mechanisms. In the fiber treatment zone, the crosslinkingsubstance can be applied to the mat via any suitable means including,but not limited to, spraying, roller coating, and a combination ofspraying and roller coating. The applicator that applies thecrosslinking agent is preferably a shower spray followed by animpregnation roller that presses the crosslinking substance into themat. In especially preferred embodiments, the shower spray includes apair of opposing shower spray applicators that direct droplets of thecrosslinking agent toward the two opposing face of the mat. Theimpregnation roller is preferably a pair of opposing rollers thatcooperatively exert 1-50 psi, preferably 1-2 psi, impregnation pressureon the mat. In particularly disclosed embodiments, the shower sprayapplicators are positioned vertically over a fiberizer inlet, and theimpregnation rollers abut the mat between the spray applicators and thefiberizer outlet. The area between the rollers can form a flooded nipthat diminishes flow of excess crosslinking agents into the fiberizeroutlet.

The dryer of the apparatus preferably includes a drying zone for formingdried fibers, and a curing zone for curing the crosslinking substanceson the dried fibers. The drying zone preferably includes the expansionchamber, which has an inlet for receiving the individual cellulosefibers from the restricted diameter conduit. The dryer inlet has atemperature within the range of about 200-315° C. so as to flashevaporate moisture from and expand the cellulose fibers. The subsequentcuring zone has an outlet through which the dried and cured fibers aredelivered from the dryer. The outlet of the curing zone preferably has atemperature within a range of about 140-180° C.

The drying and curing zones preferably comprise a first and a secondtower, respectively, in which the fibers are lofted to ensure thoroughfiber separation. In the dryer, flash drying of the fibers occurs whichmicroscopically explosively separates fibers loosely adhering togetherin the form of a fiber knot.

The fiberizer apparatus comprises at least an attrition device whichproduces a low nit level fiber output. The fiberizer may also optionallyinclude a disk refiner of conventional design coupled to the attritiondevice and a fluff generator of novel design coupled to the diskrefiner.

A preferred embodiment of the attrition device comprises a substantiallycylindrical rotor rotatable about a longitudinal axis and a housingsurrounding the rotor. The housing may include up to six mat feederassemblies each capable of simultaneously urging a wet or dry treatedmat into engagement with the rotating rotor. The rotor includes groupsor stacks of hammers extending longitudinally and radially over thesurface of the rotor, such as in an alternating fashion. In a specificarrangement, any hammer group is longitudinally and radially adjacent anempty space large enough to accept a hammer group, and any said emptyspace is adjacent a hammer group. Air flow may be directed within theattrition device away from the ends and toward the center of the rotortherein to minimize the possible accumulation of fibers at such endlocations. Also, the attrition device may include a fluid, andpreferably a liquid, flushing mechanism for use in cleaning anyaccumulated fiber from the attrition device. The attrition devicesubstantially lacks internal hot spots and dead spaces, therebyinhibiting formation of nits in the fibers produced by said device.Also, the attrition device inhibits fiber breakage.

A preferred embodiment of the fluff generator comprises three rotorshaving coplanar parallel longitudinal axes each surrounded by acylindrical housing. The rotor housings are contiguous and partiallyintersecting. All three rotors rotate synchronously in the samedirection about their axes. Each rotor comprises multiple longitudinallyextended groups of multiple radially projecting pins which, duringrotation of the rotor, travel past multiple, longitudinally extendedgroups of multiple shorter pins projecting from the inside of thecorresponding rotor housing toward the rotor axis. The fluff generatoris effective for providing additional comminution, if required, of thefibers, particularly of residual knots in the comminuted fibers producedby the attrition device.

The present invention also includes a method of producing crosslinkedcellulose fibers by applying a crosslinking substance to a mat ofcellulose fibers at a fiber treatment zone, then conveying the mat fromthe fiber treatment zone directly into a fiberizer without stopping tocure the crosslinking substance. The fiberizer separates the fibers byhammering them into substantially unbroken individual cellulose fibers,preferably having a nit level of no more than about 3. The separatedfibers are then dried at a temperature of about 200-315° C. so as toflash evaporate water from the fiber output, and then cured at atemperature of about 140-180° C.

In particularly preferred embodiments, a thermobonding agent is added tothe dried and cured cellulose fibers produced by the foregoing process.The mixture of thermobonding agent and cellulose fibers forms a mixturethat is made into a web, and the web is then heated to a sufficienttemperature to thermobond the fibers together and increase the wetstrength of the web. The thermobonding agent can, for example, be abicomponent fiber having a core component in a sheath, wherein the corecomponent (for example, polypropylene) has a higher melting point thanthe sheath component (for example, polyethylene). The thermobondingagent comprises 5-50% by weight, more preferably 20-40% of the mixture.The thermobonded crosslinked fibers have been found to produce a bulky,crosslinked product having good absorbent properties.

When the core of the bicomponent fiber is made of polypropylene and thesheath is made of polyethylene, the mixture is heated to about 130-150°C. to melt the polyethylene sheath, while allowing the polypropylenecore to maintain its structural integrity. The intact core provides amatrix in the mat that enhances the wet strength of the resulting web.The enhanced wet strength of the web permits formation of celluloseproducts using a wet laid process. These products may be impregnatedwith binders, such as carboxymethylcellulose. Soft, flexible packagingmaterials can be made by impregnating the product with a binder such asacrylic or vinyl acetate latex. Flame-retardant cellulose materials canbe made by impregnating the mat with polyvinyl chloride orpolyvinylidine chloride, for use as insulation material in buildings.Rigid packaging or soundproofing aterials can similarly be made byimpregnating the wet laid mat with acrylic or polyvinylacetate binders.

In yet another embodiment, the dried and cured fibers of the presentinvention can be collected and introduced into a pulp furnish toincrease bulk and subsequent impregnation of chemicals into a sheet madefrom the pulp furnish. The high bulk fibers of the present inventionincrease the porosity and absorbance of a sheet made from the furnish.Compared to a standard fiber made without addition of high bulk fibers,the pulp furnish of the present invention produces a sheet having anincreased saturation rate, increased bulk, and under certain conditionsenhanced strength at higher bulk. Enhanced absorbency of the resultingsheeted product increases impregnation efficiency and reduces wasteduring subsequent impregnation of the fibrous material with binders suchas latex. In particularly preferred embodiments, the high bulk fibers ofthe present invention can be added to a pulp furnish, which is then usedto make fibers that are subjected to the crosslinking process of thepresent invention.

The crosslinking agents of the present invention can includepolycarboxylic acids and urea derivatives consisting of methylolatedurea, methylolated cyclic ureas, lower alkyl substituted cyclic ureas,dihydroxy cyclic ureas, lower alkyl substituted cyclic ureas, andmethylolated dihydroxy cyclic ureas; acid anhydrides from the groupconsisting of maleic anhydride, phthalic anhydride, 4-carboxyphthalicanhydride, pyromellitic anhydride, and mellitic anhydride, pyromelliticanhydride, and mellitic anhydride; polycarboxylic acids; dialdehydes;and mixtures thereof. The crosslinking agent is more preferably apolycarboxylic acid, for example a tricarboxylic or tetracarboxylicacid, such as citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) or1,2,3,4-butanecarboxylic acid.

The pH of the cellulose fibers remains above about 2 after thecrosslinking agent is applied to the mat, and is preferably no more thanabout 4. The most preferred pH range is 3-4. A pH below 2 may damage thecellulose fibers by acid hydrolysis, whereas a pH above 4 may reduce theefficiency of the crosslinking reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of the components of the apparatus ofthe present invention.

FIG. 2 is an isometric external view of a preferred embodiment of anattrition device, where certain details of the mat feeder assemblieshave been omitted for clarity.

FIG. 3 is a transverse sectional view of a mat feeder assembly of thepreferred embodiment of the attrition device.

FIG. 4 is an isometric view of the rotor of the attrition device of FIG.2.

FIG. 5 is a plan view of a hammer plate used in the rotor of FIG. 4.

FIG. 6 is an isometric view of a stack of hammer plates used in therotor of FIG. 4.

FIG. 7 is an isometric view of the exterior of a preferred embodiment ofa fluff generator included as an option in the apparatus of the presentinvention.

FIG. 8 is a transverse sectional view through a housing portion androtor of the fluff generator of FIG. 7.

FIG. 9 is a plan sectional view of the fluff generator of FIG. 7.

FIG. 10 is an enlarged view of the crosslinking applicator portion ofthe diagram of FIG. 1, only one feeder roll being shown for simplicity.

FIG. 11 is an alternative view of the system showing a retention binthat allows curing of the crosslinking agent after the fiber is dried.

FIGS. 12 and 13 are cross-sectional views of concentric and acentricbicomponent fibers for incorporation into fibrous mats containing theindividualized fibers produced by the method of the present invention.

FIG. 14 shows the fibrous mat of the present invention containing longbicomponent fibers (prior to thermobonding).

FIG. 15 shows a matrix structure formed by the bicomponent structures inthe mat after thermobonding.

FIG. 16 is a schematic view of air laid equipment for producing thethermobonded absorbent material of the present invention.

DETAILED DESCRIPTION Overall System

The apparatus 10 (FIG. 1) of the present invention comprises a conveyingdevice 12 for transporting a mat 14 of cellulose fibers or other fibersthrough a fiber treatment zone 16; an applicator 18 for applying atreatment substance such as a crosslinking substance from a source 19thereof to the mat 14 at the fiber treatment zone 16; a novel type offiberizer 20 for completely separating the individual cellulose fiberscomprising the mat 14 to form a fiber output comprising substantiallyunbroken cellulose fibers substantially without nits or knots; and adryer 22 coupled to the fiberizer for flash-evaporating residualmoisture from the fiber output and for curing the crosslinkingsubstance, thereby forming dried and cured cellulose fibers. Theapparatus 10 of the present invention has been observed to consistentlyproduce fibers with a nit level of less than three, which issubstantially lower than obtainable using any apparatus presently knownin the art.

Raw Materials

As used herein, a “mat” denotes any non-woven sheetlike structurecomprising cellulose fibers or other fibers that are not covalentlybonded together. The fibers may be obtained from wood pulp or othersource including cotton “rag”, hemp, grasses, cane, husks, cornstalks,or any other suitable source of cellulose fiber that can be laid into asheet.

Preferably, the mat 14 includes a debonding agent which can be appliedafter formation of the mat 14 or added to cellulose fibers beforeforming the mat therefrom. For example, with mats comprising pulpfibers, the debonding agent can be added to wet pulp before the mat islaid using conventional papermaking machinery. Debonding agents tend tominimize interfiber bonds between fibers of the mat. A fair, butnonexhaustive, sampling of debonding agents is disclosed in U.S. Pat.Nos. 3,395,708 and 3,544,862 to Hervey, et al.; U.S. Pat. No. 4,144,122to Emanuelsson, et al.; U.S. Pat. No. 3,677,886 to Forssblad, et al.;U.S. Pat. No. 4,351,699 to Osborne III; U.S. Pat. No. 4,476,323 toHellsten, et al.; and U.S. Pat. No. 4,303,471 to Laursen, all of whichare herein incorporated by reference. Any suitable debonding agents maybe used, such as preferably Berocell 584 from Berol Chemicals,Incorporated of Metairie, Louisiana in a 0.25% weight of debonder toweight of fiber. However, use of a debonding agent is not required forcomplete fiberization using the present apparatus.

The mat 14 of cellulose fibers is preferably in an extended sheet formstored in the form of a roll 24 until use. While the mat 14 can also beone of a number of baled sheets (not shown) of discrete size, rolls 24are generally more economically adaptable to a continuous process. Thecellulose fibers in the mat 14 should be in a non-woven configurationproduced by a pulping process or the like, such as in a paper mill, andcan be bleached or unbleached. The mat 14 can have any of a wide varietyof basis weights. For simplicity, FIG. 1 shows a roll 24 as the sourceof each mat 14, but it is to be understood that the mat 14 can besupplied in any form amenable for storing sheet-like structures. Also,the mat may be obtained directly from the headbox of paper makingequipment or otherwise formed in any suitable manner.

It is normally not necessary that the cellulose fibers comprising themat 14 be completely dry. Since cellulose is a hydrophilic substance,molecules thereof will typically have a certain level of residualmoisture, even after air drying. The level of residual moisture isgenerally 10% w/w or less, which is not detectable as “wetness.”

FIG. 1 also shows that more than one supply, such as multiple rolls 24,of the mat 14 of cellulosic fibers can be simultaneously processed usingthe present invention. For simplicity, FIG. 1 shows two rolls 24 beingprocessed, but it is to be understood that even more supplies ofcellulosic fibers can be simultaneously processed, depending upon thecapacity of the equipment, particularly the fiberizer 20. As discussedherein below, the preferred embodiment of the fiberizer 20 can fiberizeup to six mats at one time.

At the fiber treatment zone 16, sprayers or other applicators 18 applychemicals such as crosslinking agents to the mat. Typically, chemicalsare applied uniformly to both sides of the mat. FIG. 10 shows oneparticular embodiment of the applicator in which a pair of opposingsprayers 18 are positioned adjacent each face of mat 14 to spraycrosslinking agent at the mat and saturate it with the crosslinkingagent. The wetted mat passes between a pair of impregnation rollers 28which assist in distributing the chemicals uniformly through the mat.Rolers 28 cooperatively apply light pressure on the mat (for example,1-2 psi) to force crosslinking agents uniformly into the interior of themat across its width. The rollers 28 form a seal with the mat such thatthe crosslinking agent can form a puddle at the nip. This seal helpsprevent the liquid crosslinking agent from falling into the inlet 34 aof the fiberizer 32 that is positioned vertically below the flooded nip.Other applicators may also, of course, be used. Examples of otherapplicators include size presses, nip presses, blade applicator systemsand foam applicators.

Each mat 14 is urged by the first and second pair of rollers 26, 28through the fiber treatment zone 16 where the mat 14 is impregnated witha liquid crosslinking substance. The crosslinking substance ispreferably applied to one or both surfaces of the mat using any of avariety of methods known in the art useful for such a purpose, such asspraying, rolling, dipping, or an analogous method. Spraying has theadvantage of consistent and rapid full coverage of a planar surface suchas a mat at a controllable rate, especially when the spray is applied toa surface moving past a spray nozzle or analogous applicator at a fixedrate. Roller applicators have also proven to be reliable and effectivein such applications as paper coating and the like and would thereforebe effective for applying the crosslinking substance in the presentinstance. Combinations of spray and roller applicators can also beemployed.

The crosslinking substance is typically applied in an amount rangingfrom about 2 kg to about 200 kg chemical per ton of cellulose fiber andpreferably about 20 kg to about 100 kg chemical per ton of cellulosefiber.

The rollers 28 can be positioned relative to each other to have adefined gap therebetween so as to enable them to impart a controlledsqueeze action to the impregnated mat as it departs the fiber treatmentzone 16. As mentioned above, such squeezing action facilitates completeand uniform penetration of the crosslinking substance throughout thethickness dimension of the mat. The squeezing action also helps toregulate the degree of saturation (“loading level”) of the mat 14 withthe crosslinking substance.

The crosslinking substance is a liquid solution containing any of avariety of crosslinking solutes known in the art. If required, thecrosslinking substance can include a catalyst to accelerate the bondingreactions between molecules of the crosslinking substance and cellulosemolecules. However, many if not most crosslinking substances do notrequire a catalyst.

Preferred types of crosslinking substances are selected from the groupconsisting of urea derivatives such as methylolated urea, methylolatedcyclic ureas, methylolated lower alkyl substituted cyclic ureas,dihydroxy cyclic ureas, lower alkyl substituted dihydroxy cyclic ureas,and methylolated dihydroxy cyclic ureas; acid anhydrides from the groupconsisting of maleic anhydride, phthalic anhydride, 4-carboxyphthalicanhydride, pyromellitic anhydride, and mellitic anhydride;polycarboxylic acids; dialdehydes; and mixtures thereof. A specificallypreferred crosslinking substance would be dimethyloldihydroxyethyleneurea (DMDHEU). In addition, another preferred crosslinking agent is apolycarboxylic acid, such as citric acid(2-hydroxy-1,2,3-propanetricarboxylic acid) or the1,2,3,4-butanecarboxylic acid disclosed in co-pending United Statespatent application Ser. No. 07/395,208 filed Aug. 17, 1989, which is acontinuation-in-part of Ser. No. 07/284,885, filed Dec. 15, 1988, whichis a continuation-in-part of Ser. No. 07/140,922, filed Dec. 28, 1987,which is a continuation-in-part of Ser. No. 07/004,729, filed Jan. 20,1987. Other crosslinking materials are known in the art, such asdescribed in the previously mentioned Chung patent; U.S. Pat. No.4,935,022 to Lash et al.; U.S. Pat. No. 4,889,595, to Herron et al.;U.S. Pat. No. 3,819,470 to Shaw et al.; U.S. Pat. No. 3,658,613 toSteiger et al.; U.S. Pat. No. 4,822,453 to Dean et al.; and U.S. Pat.No. 4,853,086 to Graef et al., all of which are hereby incorporatedherein by reference.

Suitable catalysts include acidic salts which can be useful whenurea-based crosslinking substances are used. Such salts include ammoniumchloride, annnonium sulfate, aluminum chloride, magnesium chloride, ormixtures of these or other similar compounds. Alkali metal salts ofphosphorous-containing acids may also be used.

In FIG. 1, the crosslinking substance applied to the mat 14 is obtainedfrom a supply 19 thereof such as a tank or analogous vessel. It is alsopossible for the supply 19 of crosslinking substance to be continuouslyproduced on-line to prevent pre-cure of the crosslinking substance thatmay occur over time if it were stored in a large vessel. On-lineproduction of the crosslinking substance is particularly advantageouswhen it contains a catalyst. Alternatively, for example, a batch of thecrosslinking substance can be prepared fresh each day, so long as nosignificant deterioration of the solution will occur during the periodin which the batch is consumed.

Crosslinked cellulose fibers are individual fibers each comprisingmultiple cellulose molecules where at least a portion of the hydroxylgroups on the cellulose molecules have been covalently bonded tohydroxyl groups on neighboring cellulose molecules in the same fiber viacrosslinking reactions with extraneously added chemical reagents termed“crosslinking substances” or “crosslinking agents”. Suitablecrosslinking agents are generally of the bifunctional type which createcovalently bonded “bridges” between said neighboring hydroxyl groups.

Crosslinked cellulose fibers have particular applicability not only inwrinkle-resistant fabrics but also in materials derived from wood pulphaving one or more desirable characteristics such as high loft, lowdensity, high water absorbency, resiliency, and light weight. As aresult, crosslinked cellulose fibers are candidates for use in absorbentstructures found in disposable products such as diapers and pads. Theyare also useful for paper towelling, wiping cloths, filters, and othersimilar uses.

Despite their desirable qualities, crosslinked cellulose fibers havepreviously enjoyed limited success as a raw material. A principal reasonfor this is because the most convenient way for a manufacturer tocrosslink cellulose fibers is by application of the crosslinking agentto a cellulosic fibrous sheet or mat which must be subsequentlyfiberized (all the constituent fibers of the sheet or mat separated fromone another) before the fibers can be subjected to a step in which thecrosslinking agent is cured. If any curing occurs before the fibers arecompletely separated, interfiber bonding can occur which would make anysubsequent attempt at complete fiberization virtually impossible.

Crosslinked cellulose fibers when used in many products cannot haveexcessive amounts of certain defects known in the art as “knots” or“nits”. Knots are agglomerations of fibers remaining after an incompletefiberization of a cellulosic fibrous sheet. Nits may be defined as hard,dense agglomerations of fibers held together by the crossing substancedue to the ability of crosslinking agents to covalently bond individualfibers together (interfiber bonding). Nits are generally regarded in theart as having a surface area of about 0.04 mm² to about 2.00 mm². A nitusually has a density greater than 0.8 g/cm³, where a density of about1.1 g/cm³ is typical. The fibers comprising a nit virtually cannot beseparated from one another in a conventional fiberizing device. As aresult, these recalcitrant particles become incorporated into the finalproduct where they can cause substantial degradation of productaesthetic or functional quality. For example, nits can substantiallyreduce the absorbency, resiliency, and lot of an absorbent product. Foraesthetically sensitive products, such as high quality paper, a “nitlevel” of three or less (two or fewer nits per 6-inch diameter test“handsheet”; see Example 1) is generally regarded as a maximallyacceptable number of nits. Knots can also seriously degrade productappearance. Also, as an example of the effect of these particles onproduct performance, filters made using crosslinked fibers containingany nits and knots would in many cases be incapable of performing tospecifications.

Conveying Device

Referring further to FIG. 1, each mat 14 of cellulosic fibers isconveyed by a conveying device 12, which can comprise, for example, aconveyor belt or a series of driven rollers with the mat positionedtherebetween. The conveying device 12 carries the mats through the fibertreatment zone 16. FIG. 1 also shows a further portion of one type ofconveying device comprised of a first pair of rollers 26 and a secondpair of rollers 28 for each mat 14. The first and second pair of rollers26, 28 are particularly effective for urging the corresponding mat at asubstantially constant and controlled rate of speed.

Fiberizer

The subsystem following the fiber treatment zone is a fiberizer 20 whichserves to comminute one or more mats 30 impregnated with thecrosslinking substance into individual substantially unbroken cellulosefibers comprising a fiber output. The fiberizer 20 performs its task onone or more mats, which are preferably still moist (but which may bedry) from application of the crosslinking agent. In this case, the wetsheets are delivered directly and immediately to the fiberizer by theconveyor 12 without aging or other significant delays. As detailedbelow, the preferred embodiment of the fiberizer 20 is designed tominimize interfiber bonding and the formation of nits therein. Also, thepreferred embodiment of the fiberizer 20 thoroughly fiberizes eachimpregnated mat 30, thereby virtually eliminating residual knots.

The preferred embodiment of the fiberizer 20 comprises an attritiondevice 32 as detailed hereinbelow and in copending U.S. patentapplication Ser. No. 07/607,312 entitled “Fiberzing Apparatus”, filed onOct. 31, 1990, which is incorporated herein by reference. The attritiondevice 32 preferably can simultaneously fiberize a plurality ofimpregnated mats 30 and has a separate mat inlet 34 a, 34 b forreceiving each corresponding impregnated mat.

The exterior of a preferred embodiment of the attrition device 50 isshown in FIG. 2, which comprises an elongated cylinder-shaped housing 52having an exterior surface 54. A first end panel 56 is located on oneend of the housing 52 and a second end panel 58 is located at the otherend of the housing 52. Multiple mat inlets (two of which 60 a, 60 b areshown) defined by the housing are located radially in an arc comprisinga portion of the circumference of the housing 52, where each mat inletis dedicated to feeding a separate mat into the attrition device 50. Anoutlet chute 62 extends from the housing 52. Each end panel 56, 58defines a central orifice 64 through which coaxially extends acorresponding rotor shaft end 66 rotatable relative to the housing 52.One rotor shaft end 66 is coupled to a drive motor 68 serving to impartrotational motion thereto.

An air flow port 69 is provided through each end panel 56, 58. As adownstream blower 160 (discussed below) coupled to outlet 62 isoperated, air is drawn in through openings 69, and around the ends ofthe rotor 100 (discussed below in connection with FIG. 4) to assist inminimizing the accumulations of fiber at such locations. Althoughvariable, air typically flows at a rate of about 50 m³/min through eachof the openings 69. Also, a conduit (not shown) is typically includedand coupled to wall 52 delivering water or other cleaning fluid to theinterior of the housing through plural nozzle openings to clean anyfiber accumulations from the attrition device. A liquid cleaningoperation is typically accomplished by directing water toward the rotorsin a direction somewhat counter to the direction of the normal rotorrotation as the rotor is rotated in this direction. Cleaning may beperiodically performed, such as once every sixteen hours of operation ofthe attrition device, depending in part upon the volume of fiber beingprocessed. By cleaning fiber accumulations in this manner, theaccumulations do not end up in the finished product where they maycomprise bonded nits.

Each mat inlet includes a feeder assembly, such as assemblies 70 a-70 fshown partially in FIG. 2, each mounted exteriorly relative to thecylindrical housing 52 at a location adjacent the corresponding matinlet. A representative feeder assembly (such as 70 d in FIG. 2) isshown in more detail in the transverse sectional view of FIG. 3. Eachfeeder assembly 70 is comprised of a first feed or seal roller 72 and asecond feed or seal roller 74 extending longitudinally between the firstand second end panels 56, 68 (FIG. 2). Also extending longitudinallybetween the first and second end panels 56, 58 are corresponding supportangles or brackets (such as 76 a and 76 b in FIG. 3) and wedge-shapedalignment or mounting bars (such as 78 a and 78 b in FIG. 3). Since FIG.3 only depicts one feeder assembly 70 d, angles 76 a and 76 b correspondto the feeder assembly 70 d. The first and second seal rollers 72, 74extend longitudinally in a direction substantially parallel to, and havea length substantially equal to, the corresponding mat inlet 60 asituated between a leg 80 of the angle bracket 76 a and a leg 82 of theangle bracket 76 b. The seal rollers 72, 74 are rotatably mounted forrotation about their respective longitudinal axes 84, 86 at locationsequidistant from the mat inlet 60 a. The distance D, from a planethrough the axes of seal rollers 72, 74 to the effective rotor surface144 swept by the hammers of the rotor 100 (FIG. 4) is preferably fromabout one-half inch to no more than about four inches when wet sheetsare being fed to the rotor 100. This minimizes the possibility ofplugging of the opening 60 a as the sheets are being delivered thereto.

In one specifically preferred design, each seal roller has a centralshaft and an outer roll. The ends of the central shaft of each sealroller 74 are coupled by a respective bearing to the end plates 56, 58.In addition, the ends of the central shaft of the seal rollers 72 aresupported for rotation by a bracket (one being shown as 87 in FIG. 3).Typically the seal rollers are of a rigid material, such as steel, withthe seal roller 74 being mounted at a fixed location. The ends of theshaft of the seal roller 72 are positioned within respective recesses 85in the respective brackets 87. The bracket 87 may be pivotally coupledto the housing for pivoting in the direction of arrow 91 upon removal ofa bolt or other stop 89. When bracket 87 is shifted upwardly in FIG. 3,the seal roller 72 may be removed for repair and or cleaning and toprovide access to seal roller 74. Pneumatic cylinders, not shown,typically apply a load of from 5 psi to 80 psi to the respective ends ofthe shaft of the seal roller 72 to bias the seal rollers together. Thispressure is typically relieved to allow the feeding of a sheet betweenthe seal rollers and is then reinstated during normal operation of theattrition device. At least one of the seal rollers, such as roller 74,is rotatably driven via a motor (not shown) at a controlled angularvelocity to advance a mat (not shown) situated between the first andsecond rollers 72, 74. Roller 74 may, for example, be driven in thedirection of arrow 93 at a predetermined mat feed rate through the matinlet 60 a.

A first guide 88 and a second guide 90 are also mounted to thecorresponding mounting brackets 76 a and 76 b, respectively. Each of theguides 88, 90 extend longitudinally in a direction substantiallyparallel to the corresponding seal rollers 72, 74, respectively. Eachguide 88, 90 is typically constructed of a rigid material and includesan outer edge 92, 94, respectively, adjacent to, but spaced from thesurface of the corresponding seal roller 72, 74, respectively, along thefull length of the roller. The guides 88, 90 thereby serve tosubstantially prevent air from passing past the guides and to thecorresponding mat inlet 60 a. Therefore, substantially all of the airdrawn into the attrition device passes through the openings 69 (aspreviously explained). The fiber mat passing through inlet 60 a passesan optional nose bar 95 and is delivered against the rotor 100traversing the effective rotor surface 144 (FIG. 3). The gap between theinlet 60 a and the effective rotor surface is typically no more thanabout one-fourth to one inch.

FIG. 4 shows a rotor 100 of the type coaxially mounted inside theattrition device housing 52 (FIG. 1). The rotor 100 comprises aplurality of substantially annular spacer or hammer mounting plates 102mounted to the rotor shaft 104. The plates 102 extend radially outwardlyfrom the longitudinal axis “A” of the rotor shaft 104 and are parallelto one another. The rotor 100 also has a first rotor end plate 106 and asecond rotor end plate 108, each substantially annular in shape andoriented parallel to the mounting plates 102. The first and second rotorend plates 106, 108 are mounted coaxially to the rotor shaft 104 andhave a diameter sufficiently large such that only a narrow gap (e.g.one-sixteenth to one-half inch) is left between the inner surface of thecylindrical housing (not shown in FIG. 4) and the perimeter of the firstand second end plates 106, 108. The illustrated plates 106, 108 extendradially outwardly beyond the distal ends of hammers 116 to minimize thepossible accumulation of fibers adjacent to the end plates.

Attached to and extending between the first and second end plates areplural inner mounting rods 110 and an identical number of outer mountingrods 112 oriented parallel to the longitudinal axis “A” of the rotorshaft 104. The inner and outer mounting rods 110, 112 are secured to thefirst and second rotor end plates 106, 108. As shown clearly in FIG. 4,the mounting rods 110, 112 are arranged as plural equiangularly spacedpairs. Each pair comprises a single inner mounting rod 110 and aradially outwardly positioned single outer mounting rod 112. A typicalrotor 100 has sixteen such pairs of rods arranged radially about therotor axis “A”.

Each pair of mounting rods 110, 112 has mounted thereto plural groups ofhammer plates, each group comprising a hammer assembly 116. Each suchhammer assembly 116 is located either between adjacent mounting plates102 or between a spacing plate 102 and an adjacent rotor end plate 106,108. However, each hammer assembly 116 is spaced from an adjacent hammerassembly 116, by an empty space 118 large enough to accommodate anotherhammer assembly. As a result, on a rotor 100 with twenty-seven mountingplates 102 and two rotor end plates 106, 108, for example, the maximalnumber of hammer assemblies 116 held by a given pair of mounting rods110, 112 is fourteen.

A representative flat hammer plate 130 of assembly 116 is depicted inFIG. 5, wherein each hammer plate 130 has a proximal end 132 positionedtoward the rotor axis (not shown) and a distal end 134 positionedradially outward relative to the rotor axis. The hammer plate 130 alsodefines two mounting holes 136 a, 136 b for attaching the hammer 130 toan associated pair of mounting rods 110, 112 (not shown in FIG. 5). Thedistal end surface 134 of the hammer plate has a trailing edge 138 and aleading edge 140, wherein the leading edge 140 extends radially outwardrelative to the rotor axis beyond the trailing edge 138. The distal end134 is cut at a five degree angle relative to a line 142 parallel to theproximal edge 132. The direction of rotation of the rotor is indicatedby an arrow 145 in FIG. 5.

As shown in FIG. 6, each illustrated hammer assembly 116 comprisesplural planar plate-like hammers 130 (three being shown in this figure).These plates are typically spaced apart by spacers (not shown). Each ofsaid hammer assemblies 116 located between adjacent mounting plates 102(only one plate 102 being shown in this figure) also includes aleft-angled hammer 146 and a right-angled hammer 148 each having a lip150 a, 150 b, respectively, extending transversely in an opposingdirection relative to each other. The width dimension 152 of the lip ofeach angled hammer is typically equal to or slightly less than half thethickness dimension of a mounting plate 102. Also, each of the hammerassemblies 116 located between a plate 102 and a rotor end platereplaces one of the L-shaped hammers with a flat hammer plate adjacentto the end plate. Other hammer configurations and arrangements may beused. However, a preferred hammer arrangement minimizes any gaps in thesurface swept by hammer elements to preferably no more than one-fourthof an inch.

The illustrated embodiment 50 of the attrition device is operated bydriving the rotor 100 at a high angular velocity while feeding one ormore impregnated mats through one or more corresponding mat inputs. Themat is urged at a controlled linear velocity into the corresponding matinput slot 60 by the controlled rotation of the feed rollers 72, 74. Asthe impregnated mat enters a mat inlet, it is repeatedly impacted by thedistal end surface and in particular, the leading edge of the hammerplates, which effectively and completely comminutes the mat into itsindividual constituent fibers, substantially free of knots and nits.

The preferred embodiment 50 of the attrition device as describedhereinabove is particularly effective in simultaneously fiberizing oneor more separate mats (up to six) to form a volume of individualizedcellulose fibers having a nit level substantially lower than levelsachievable with existing attrition devices such as hammermills. This isbelieved to be due to the fact that the present attrition device lackshot spots and dead spaces, wherein fibers can accumulate, found inconventional hammermills or other attrition devices currently used inthe art.

Referring further to FIG. 1, a first conveyor fan 160 of conventionaldesign can be utilized for propelling the fibers from the outlet 62 ofthe attrition device 32 through a conduit 162.

An optional component of the fiberizer 20 is a first cyclone 164 orsimilar apparatus known in the art utilized in a conventional manner toconcentrate the fibers passing out of the outlet 62 of the attritiondevice 32. The first cyclone 164 receives the fibers through the conduit162 coupled thereto.

Excess air can be recovered at the top 166 of the first cyclone 164 andrecycled as required through a conduit to a location upstream of thefirst conveyor fan 160 (if used). Such additional air can be beneficialfor easing the transfer of the fibers through the first conveyor fan160.

A disk refiner 168 is another optional component of the fiberizer 20which can be employed to effect additional separation of fibers (removalof knots) if required. The disk refiner 168 is of a type known in theart and comprises a disk refiner inlet 170 and a disk refiner outlet172. A representative disk refiner 168 is type DM36 manufactured bySprout-Bauer, Incorporated of Muncie, Pa. If the disk refiner 168 isused, the inlet 170 thereof is coupled via a conduit 174 to an outlet176 of the first cyclone 164.

A second conveyor fan 178 may optionally be utilized to urge propagationof the fibers through a conduit 180 downstream of the disk refiner 168.Excess air can be recovered from the top 166 of the first cyclone 164and routed via a conduit 181 to a tee 182 just upstream of the secondconveyor fan 178.

Another optional component of the fiberizer 20 is a fluff generator 190which receives the fibers from the optional second conveyor fan 178through a conduit 184. The fluff generator is described in detail belowand in copending U.S. patent application Ser. No. 07/607,157 entitled“Multi Pin Rotor Fiber Fluff Generator”, filed on Oct. 31, 1990,incorporated herein by reference.

Referring now to FIG. 7, a preferred embodiment of the fluff generator190 comprises a housing 192 shaped in the form of three contiguous,partially intersecting cylinders, including a first housing portion 194opening into a second (or middle) housing portion 196, which opens intoa third housing portion 198. Each housing portion 194, 196, 198 has alongitudinal coplanar axis A1, A2, A3, respectively. The housing 192 hasan inlet 200 permitting delivery (arrow 202) of fibers to the firsthousing portion 194, and an outlet 204 conducting fluffed fibers away(arrow 206) from the third housing portion 198.

As shown in FIG. 8, showing a transverse sectional view of the firsthousing portion 194, the interior surfaces 212 of each of the first,second, and third housing portions have affixed thereto multiple statorpins 214 radially pointing toward the respective axis of the housingportion. The pins 214 are grouped in longitudinally extended rows alonglines parallel to the respective housing portion axis.

Each of the first, second, and third housing portions 194, 196, 198,respectively, is in surrounding relationship to a first rotor 216, asecond rotor 218, and a third rotor 220, respectively, as shown in FIG.9. Each rotor 216, 218, 220 has a corresponding rotor shaft 222, 224,226, coaxial with the axis A1, A2, A3 of the respective housing portion.As shown in FIG. 8 (showing a transverse sectional view of the firsthousing portion 194 only, but applicable to illustrate similar detailsinside the second housing portion 196 and third housing portion 198) andFIG. 9, to the shaft 222 of the rotor 216 are mounted fourlongitudinally extended rows of plural rotor pins 228, where each row ofrotor pins 228 is equiangularly spaced in a radial manner around thecorresponding rotor shaft 222. The rotor pins 228 radially extend fromthe shaft 222 nearly to the inside surface 212 of the correspondinghousing portion 194 but are positioned on the rotor shaft 222 such thatthey will pass between longitudinally adjacent stator pins 214 when therotor 216 is rotating about its axis. Rotor shafts 224 and 226 aresimilarly equipped with rotor pins 228.

As shown in FIG. 9, each rotor shaft 222, 224, 226 has a first end 230,232, 234, respectively, and a second end, 236, 238, 240, respectively,each extending through and journaled in the corresponding housingportion 194, 196, 198, respectively. The first and second ends of eachrotor shaft extend outside the corresponding housing portion. A pulley242 a, 242 b is attached to each of the first ends 230, 232,respectively, of the first and second rotor shafts 222, 224,respectively. Likewise, a pulley 244 a, 244 b is attached to the secondends 238, 240, respectively, of the second and third rotor shafts 224,226, respectively. The first end 234 of the third rotor shaft isrotatably coupled directly or indirectly to a drive motor 250. Each setof pulleys is coupled by a drive belt 252 a, 252 b ensuring that, whenthe drive motor 250 rotates the third rotor 220, the second and firstrotors 218, 216, respectively, synchronously rotate in the samerotational direction as the third rotor 220.

The fluff generator 190 is operated by synchronously driving the rotors216, 218, 220 at a high rotational speed and conducting fibers 202 (FIG.7) from their disk refiner 168 (FIG. 1), where the velocity of saidfibers is increased via the second conveyor fan 178, into the inlet 200of the fluff generator 190. The fibers are conducted sequentiallythrough the first, second, and third housing portions 194, 196, 198,respectively, and exit 206 the fluff generator 190 through the outlet204. As the fibers pass through the housing 192 of the fluff generator190, they encounter strong agitation and turbulence generated by thegroups of rotor pins 228 on each of the three rapidly rotating rotors216, 218, 220 passing by the stationary stator pins 214. By encounteringsuch turbulence and agitation, any knots remaining in the fibers arecomminuted to form a fiber output containing virtually no knots.

As used herein, the “fiber output” is the mass of thoroughlyindividualized fibers exiting the fiberizer 20 and passing to the dryer22.

As discussed hereinabove, the disk refiner 168 and fluff generator 190are optional components of the present apparatus 10. In most cases, theattrition device 32 alone is adequate for completely fiberizing pluralmats. However, in cases where the mats are unusually bulky, the diskrefiner 168 and fluff generator 190 can be employed, particularly toensure the absence of knots in the fiber output.

Dryer

Referring further to FIG. 1, a preferred embodiment of the presentapparatus 10 includes a dryer 22 which is utilized to perform twosequential functions: remove residual moisture from the fibers and curethe crosslinking agent. Preferably, the dryer 22 comprises a dryingintroduction zone 273 for receiving fibers e.g. from fluff generatoroutlet 204 and for removing residual moisture from the fibers via a“flash drying” method and another drying zone 260, 262 for curing thecrosslinking agent. In FIG. 1, the curing starts in zone 260 andcontinues through zone 262.

The FIG. 1 embodiment shows that zone 273 is coupled to the fluffgenerator outlet by a conduit 272 and to a source 274 of heated air,typically produced by combustion of a supply of natural gas 276 andfresh air 278. The temperature of heated air is regulated to maintainthe temperature of the drying zone 273 within a range of about 200° C.to about 315° C. To achieve this temperature in zone 273, air is blownfrom source 274 at a temperature, for example, of about 260° C. Thedrying zone 273 is a J-shaped conduit that includes a necked down orreduced diameter conduit having an initial portion 273 a, and a rightangle portion 273 b that flares to increase the diameter of the conduitas it connects with the inlet 268 of the expansion chamber defined bybody 266 of drying zone 260. The diameter of the reduced diameterportion conduit is reduced compared to the diameter of the conduit 272through which the fibers flow from the fiberizer. The diameter reductionincreases the velocity of the flow of fibers and causes a decrease inpressure that promotes rapid evaporation and drying of the fibers inportion 273 a. The fiber output in conduit 272 is introduced into thereduced diameter portion 273 b of conduit 273 at inlet 275 immediatelydownstream from where portion 273 a necks begins.

As the fiber output passes into the drying zone 273 at inlet 275, thewet fibers comprising the fiber output are substantially instantaneouslyexposed to the high temperature in this zone. Such rapid exposure tohigh temperature imparts a “flash drying” effect to the fibers, therebycausing rapid and thorough drying. Such “flash drying” also tends toseparate, in a microscopically explosive manner, fibers that aretouching one another, thereby ensuring thorough separation of thefibers. The passage time through the drying zone 273 is preferably lessthan one second, which is deliberately kept short to avoid overheatingand scorching the fibers, which become highly susceptible to scorchingafter the residual moisture has been driven therefrom.

As the fibers enter the expanding throat of section 273 b and enterfirst drying zone 260, pressure changes enhance the microscopic fiberexplosions as water vapor is rapidly released from the fibers and pushesthe fibers apart. This expanding throat 273 b mates with an expandinginlet 268 to an expansion chamber defined by first drying zone 260. TheFIG. 1 embodiment shows that the first drying zone 260 comprises a firsttower 264 having a body portion 266, an inlet 268, and a first toweroutlet 270. The dryer zone 273 is coupled via conduit 272 to the outletof the fluff generator 190. Since the fluff generator 190 is an optionalcomponent, it is also possible to couple the dryer introduction zone 273directly to the outlet 62 of the attrition device 32 if neither thefluff generator 190 nor the disk refiner 168 are included.

In FIG. 1, the first tower outlet 270 is shown preferably coupled via aconduit 280 to a down tube 282, which is coupled via a conduit 284 to athird conveyor fan 286 located at an inlet 288 of a second tower 290.The third conveyor fan 286 performs the function of transporting thefibers through the dryer which thereby pass through the inlet 288 of thesecond tower 290.

The second tower 290 is shown which includes the inlet 288, a secondtower body 292, and an outlet 294 serving as an outlet of the dryer 22.Dried fibers are propelled through the inlet 288 of the second tower 290via the third conveyor fan 286. As the fibers are lofted through thesecond tower body 292, they are still exposed to a curing temperaturewithin a range of about 140° C. to about 180° C., which is sufficient toeffect curing of the crosslinking agent without scorching the dryfibers. The lofting keeps the fibers separated until the crosslinkingreaction is complete. The curing temperature depends upon the type ofcrosslinking material used to treat the fibers and also is set at alevel so as not to scorch the fibers during curing. It should be notedthat single stage dryers may also be used.

The dried and cured fibers exiting the dryer outlet 294 have anextremely low level of nits and virtually no knots. Further, they arenot discolored from scorching and the like, and have a median fiberlength substantially unchanged from the median length of the fiberscomprising the mat 14.

FIG. 1 also shows a second cyclone 300 of conventional design coupledvia a conduit 302 to the dryer outlet 294, serving to concentrate thefibers passing therethrough in preparation for collection. Excess air304 is vented through the top 306 of the second cyclone 300. Theresulting concentrated fibers can be collected using any of a number ofcollection devices 308 known in the art, such as fiber bagging devices.

It is possible to add retention bins to the system of FIG. 1 to increasecuring time of the crosslinking agent. Such a bin 310 is shown betweencyclone 300 and collection device 308. Bin 310 has the shape of aninverted pyramid, and is large enough to hold the output from cyclone300 for a period of 1-5 minutes when the system is operating. The bin ispreferably heated to a temperature of about 175-190° C. to promotecuring of the crosslinker. The most preferred temperature within thisrange may vary with the capacity of the bin, because larger retainingbins will allow longer residence times at lower temperatures. A bin witha sufficient capacity to collect 5 minutes of output from cyclone 300could, for example, be heated to a temperature of 175° C. A smaller binwith only a three minute capacity, however, may require a residencetemperature of 180° C. Very small retention bins, for example bins witha one minute capacity, may have a bin temperature of 190° C. to promotecuring of the crosslinking agent in the shorter period of time.

An alternative crosslinking system having a retention bin is shown inFIG. 11, in which an air heater 400 propels fibers through a conduit402. The fibers which enter conduit 402 have already been saturated withthe crosslinking agent, and the fibers begin to dry as they are movedthrough heated conduit 402 by a conveyor fan 404 into a conduit 405.Fibers are then introduced tangentially from conduit 405 into a topcylindrical area of a first dryer 406, and then fall downwardly. Thebottom portion of dryer 406 tapers at 408 to an outlet 410 such thatpartially dried fibers are withdrawn through a conduit 412 and propelledby a second heated air blower 414 through a conduit 416. Movement ofpartially dried fibers through conduit 416 is assisted by a secondconveyor fan 418, which helps propel the partially dried fiberstangentially into a top cylindrical zone of a second dryer 420. Dryer420 tapers at 422 to a restricted outlet 424 through which fibers arewithdrawn through a conduit 426 and emptied into a retention bin 428.Bin 428 has a tapering bottom portion 430 with a selectively closedoutlet 432 through which cured fibers may be withdrawn from theretention bin at preselected intervals when the bin is filled tocapacity. When outlet 432 is opened, fibers are introduced into aconduit 434, whence they are conveyed by a conveyor fan 436. Heated airis not introduced into conduit 434, hence cooling of the fibers occursas they travel through conduit 434 and are collected in a collection bin438.

EXAMPLE I

In one specific embodiment of the invention, the fibers are movedthrough conduits 402, 405 at a temperature of about 250° C. First dryer406 is approximately 50 feet tall, and has a largest diameter of 14 feetin its cylindrical upper portion. The heated fibers undergo a flashevaporation as they enter the relatively low pressure environment oflarge diameter tank 406. The partially dried fibers then exit throughconduit 412 at a temperature of about 60° C., and are once again heatedto about 250° C. by hot air from air heater 414 blowing through conduit416. Flash evaporation of the fibers once again occurs in dryer 420,which in this example is approximately 60 feet tall and has a largestdiameter of about 16 feet in its cylindrical upper portion. The flashdried fibers exit dryer 420 through outlet 424 and are collected inretention bin 428. The bin has a sufficient capacity to collect fiberoutput from dryer 420 for a period of 60 seconds. The retention bin ismaintained at a sufficient temperature, for example 190° C., to allowthe crosslinking agent to cure during the 60-second period of retentionin bin 438. At the end of a 60-second retention period, the fibers arewithdrawn through outlet 432 and conveyed through conduit 434 to holdingtank 436.

EXAMPLE II

In this example, non-woven fibrous mats were impregnated with acrosslinking agent, fiberized, dried, and cured using the apparatus asdiagrammed schematically in FIG. 1.

Two 52-inch wide mats of southern pine kraft wood pulp fibers (typeNB316 from Weyerhaeuser Company) and having a basis weight of 680 g/m²were fed to the apparatus. The mats were impregnated usingdimethyloldihydroxy-ethylene urea at a concentration of about 5%,applied over both sides of each mat using a combination of spray nozzlesand impregnation rollers. The loading level of crosslinking agent wasabout 4.5% w/w.

The treated fiber mats were fed at the rate of 8 meters/minute to theattrition device 32. The specific attrition device used in this examplewas equipped with six mat inlets and a rotor having 16 rows of hammersas described above around the circumference of the rotor. The rotor hada diameter of 30 inches and was rotated at an angular velocity of 1200rpm by an electric motor. Other rpm rates have also been tested and haveproven satisfactory, including extremely high rpm rates.

Random samples of fibers were obtained from the output attrition deviceand observed for nits. These samples were 2.6 grams and wereconsistently observed to have fewer than three nits on the average withmost samples having no nits. The attrition device was flushed with wateronce every sixteen hours for cleaning purposes.

A disk refiner was employed downstream of the attrition device. Thisspecific disk refiner was a DM36 refiner as previously mentioned.

A fluff generator as described in FIGS. 7-9 was also employed in thisdownstream of the disk refiner.

The temperature at the dryer input 273 in this example was within therange of 200° C. to 315° C., and conduit 273 a had a diameter of 3½feet. The tower body 266 that formed zone 260 had a diameter of 7 feet,hence the diameter ratio of conduit 273 a to tower 266 was 1:2. Thetemperature at the second tower outlet 294 was within the range of 140°C. to 180° C.

Crosslinked fiber at the output of the dryer was produced at a rate ofabout 5000 pounds per hour and had a nit level on an average of from 1to 3 and a maximum bulk of greater than 22. Bulk and nit levels weredetermined by the following procedure, involving the production of test“handsheets” with a diameter of about 6 inches:

A “British handsheet mold” was filled with 3 to 4 inches of water. Toapproximately 750 mL of water were added 1.2 grams of pulp, availablefrom Weyerhaeuser Company, followed by agitation using a Waring blenderfor 20 seconds to yield a pulp slurry. A 2.4 gram sample of the aboveobtained crosslinked fiber was added to the pulp slurry in the blenderfollowed by agitation therein for another 10 seconds. The resultingslurry was added to the handsheet mold up to a fill mark. The slurry inthe mold was gently mixed using a spatula for 3 seconds, then drained,leaving the pulp wet laid on the screen in the mold. The wet pulp layerwas blotted to remove as much moisture as possible, then removed fromthe screen. The resulting handsheet was dried between two blotters on adrum dryer, then weighed to the nearest 0.01 gram immediately afterdrying.

Bulk was determined using a caliper, performed immediately after drying.Mean thickness was determined using five thickness determinations ofvarious locations on the handsheet. Bulk was calculated in units ofcm³/g as follows:$\frac{\left( {{mean}\quad {thickness}} \right)\quad {cm}\quad (20.38)\quad {cm}^{2}}{\left( {{Handsheet}\quad {weight}} \right)\quad {grams}} = {Bulk}$

Nit level was determined by examination of the handsheet and simpledetermination of the number of nits present on the handsheet. If no nitswere observed a nit level of 1 was assigned to the test sheet; if 1 nitwas observed, a nit level of 2 was assigned to the sheet; if 2 nits wereobserved, a nit level of 3 was assigned to the sheet; and so forth forhigher nit levels.

Therefore, the apparatus of the present invention effectively produces alow nit level product, and one of high bulk even when crosslinkingagents are used.

Fiber Thermobonding

One problem with the fiber crosslinking method of the present inventionis that it may produce a wet laid sheet having reduced tensile strength.The intrafiber crosslinking in the cellulose chemically inhibitsinterfiber bonds that give integrity to a web. As a result, it issometimes difficult to make a wet laid sheet with the dried and curedcrosslinked fibrous output of the present invention. The presentinventors have overcome this problem by adding a thermobonding agent tothe dried and cured individual cellulose fibers to form a mixture thatis made into a wet laid web. The web is then heated to a sufficienttemperature to activate the thermobonding material and increase the wetstrength of the web by providing a thermoplastic matrix within the web.Addition of a thermobonder can also be used to increase the tensilestrength of an air laid web.

A number of synthetic fibers have been developed in recent years whichare heat-adhesive (thermobondable) synthetic fibers. Thesethermobondable synthetic fibers can be used to bond fibers together,thereby providing an absorbent material with improved strength thatallows thinner and lighter weight products to be produced. Examples ofpatents describing such fibers, or their use or production, are U.S.Pat. Nos. 4,189,338 (non-woven fabric comprising side-by-sidebicomponent fibers); 4,234,655 (heat adhesive composite fibers);4,269,888 (heat adhesive composite fibers); 4,425,126 (fibrous materialusing thermoplastic synthetic fibers); 4,458,052 (absorbent materialcontaining polyolefin pulp treated with a wetting agent); 4,655,877(absorbent web structure containing short hydrophilic thermoplasticfibers); and European Patent Application No. 248,598 (polyolefin-typenonwoven fabric). Thermobonding agents are commercially available, forexample Celbond® dual-polymer (bicomponent) fibers from Hoechst CelaneseCorporation of Somerville, N.J. Examples of other commercially availablethermobonding agents include Vinyon® monocomponent fiber made by HoechstCelanese from a copolymer of polyvinyl chloride and polyvinyl acetate,and Kodel® 100% polyester monocomponent fiber from the Eastman KodakCompany of Rochester, N.Y.

The manufacture and use of thermobondable bicomponent synthetic fibersis fully disclosed in European patent application publication number340,763, filed Mar. 5, 1989, the disclosure of which is incorporated byreference. Briefly summarized, the bicomponent fiber comprises an innercore component and an outer sheath component in which the core componentis a polyolefin or polyester, the sheath component is a polyolefin, andthe core component has a higher melting point than the sheath component.The core is preferably surrounded by a coaxial sheath, in which the corecomponent typically has a melting point of at least about 150° C., whilethe sheath component typically has a melting point of about 140° C. orlower. The sheath component is preferably selected from the groupconsisting of polyethylene, polypropylene, poly(1-butene), andcopolymers and mixtures thereof, while the core component is selectedfrom the group consisting of polypropylene,poly(1,4-cyclohexylene-dimethylene-terephtalate),poly(4-methyl-1-pentene), polyester and copolymers and mixtures of theforegoing. The weight ratio of the sheath and core components in thebicomponent fiber is preferably in the range of 10:90 to 90:10, morepreferably from about 30:70 to 70:30, and most preferably from about40:60 to 65:35. The cross-section of the fiber is preferably circularand has a fineness of about 1-7 decitex. The fiber is cut to a length ofabout 3-24 millimeters, typically about 5-20 millimeters, preferablyabout 6-18 millimeters.

The wet strength of a crosslinked cellulose material in accordance withthe present invention can be increased with the thermobondablebicomponent fibers by subjecting the bicomponent fibers and crosslinkedfibers to blending. The bicomponent fibers and non-bicomponent fiberscan be blended, for example, by dispersion in water in a wet-laidnonwoven production process, so as to obtain a nonwoven web in which thebicomponent fibers are distributed in a substantially random andhomogenous manner.

The percentage weight of bicomponent fibers in the fluff is preferablyin the range of about 5-50%, more preferably 20-40%. The nonwoven webshould contain a certain minimum amount of the bicomponent fibers inorder that the improved characteristics due to the supporting structureof the thermobonded bicomponent fibers can be achieved. Thus, abicomponent fiber content of about 5% is regarded as being the usualminimum. On the other hand, the bicomponent fibers of the presentinvention need not necessarily constitute a large portion of the fluffIf a large amount of bicomponent fiber is added to the mixture, thephysical properties of the bicomponent fiber will begin to predominateover the desired characteristics of the crosslinked fiber, such as highbulk and absorbency. One of the advantages of the bicomponent fibers ofthe present invention is that they can be used in low amounts. Theweight ratio of the bicomponent fibers to the non-bicomponent fibers inthe fluff can therefore be about 5:95-10:95.

FIG. 12 shows a cross-section of a bicomponent fiber 8 with a concentricconfiguration. A core component 10 is surrounded by a sheath component12 with substantially uniform thickness, resulting in a bicomponentfiber in which the core component 12 is substantially centrally located.FIG. 13 shows a cross-section of another bicomponent fiber 14 with anacentric configuration. A core component 16 in this configuration issubstantially surrounded by a sheath component 18 with a varyingthickness, resulting in a bicomponent fiber in which the core component16 is not centrally located.

FIG. 14 shows the structure of a nonwoven web prior to thermobonding.Bicomponent fibers 20, according to the present invention, are arrangedin a substantially random and homogenous manner among non-bicomponentfibers 22 in the fluff FIG. 15 shows the same structure as illustratedin FIG. 14, but after thermobonding. The sheath component of thebicomponent fibers has been melted by the thermobonding process, fusingthe intact core components together (as at 24), thus forming asupporting three-dimensional matrix. The non-bicomponent fibers 22 arerandomly arranged in the spaces defined between the bicomponent fibers,and some of the non-bicomponent fibers 22 have been fused (as at 26) tothe bicomponent fibers.

FIG. 16 illustrates an air laid method by which the mixture ofbicomponent and non-bicomponent fibers is fused to achieve thetransition shown between FIG. 14 and FIG. 15. The mixture of cellulosefibers and thermobonding agents is introduced through an inlet 60 to afluff mat forming hood 62, where a fluff mat 63 is formed by suction ofthe fibers onto a wire mesh 64. A layer of superabsorbent polymer (SAP)may be sprinkled on top of mat 63 to form a top SAP layer or introducedabout halfway through formation of mat 63 to form a middle SAP layer.The fluff mat 63 typically passes through a series of rollers 66, inwhich the mat 63 is condensed or embossed prior to the thermobondingprocess. The mat 63 is then led via a second wire mesh 72 past a throughair oven 68, which thermobonds the material, thus producing thesupporting structure formed by the core component of the bicomponentfibers, as shown in FIG. 15. The thermobonded material is then led to aconverting machine 74, in which the production of hygiene absorbentproducts, such as diapers, takes place.

The following examples will illustrate use of Celbond® K-56 bicomponentfiber with the intrafiber crosslinked fibers of the present invention.

EXAMPLE III

A slurry was prepared containing 20% weight percent of 0.4 inch longCelbond® K-56 type bicomponent fiber, and 80% of high bulk additive(HBA) fibers, prepared as in Example II. The bicomponent and BBA fiberswere mixed in water at a consistency of approximately 0.01%. This slurrywas dewatered on the forming wire of an inclined wire Fodrinier-typewet-laid nonwoven machine, and subsequently transferred, while stillcontaining approximately 50% water, to a rotary through-dryer, whichserved to both fully dry the web and partially fuse the bicomponentCelbond® portion of the web. The basis weight of the dried web was 275gsm, with a bulk density of approximately 0.03 g/cc. A control web,formed using conventional woodpulp at the 80% level in place of the HBA,resulted in a web with a bulk density of 0.1 g/cc. The resultingHBA/Celbond® dried and fused web possesses sufficient integrity to allowhandling in further conversion steps, e.g., slitting/winding.

EXAMPLE IV

The bonded web of Example III is treated with a secondary bindertreatment in order to develop specific properties in the web whilemaintaining its desirable low-density characteristics. A saturant isprepared by diluting a suitable binder product (such as Airflex 120latex, manufactured by Air Products and Chemicals, Inc. of Allentown,Pa.) with water to a solids content of 5% from its original solidscontent of 52%. Airflex 120 is a self-crosslinking vinylacetate-ethylene polymer, possessing low Tg (−20° C.) which results inthe combination of tensile strength, abrasion resistance and flexibilityin substrates to which it is applied. The bonded web of Example III issaturated in a bath of the diluted latex, then passed through a set ofnip rollers which squeeze out excess latex, resulting in a webcontaining approximately 100% of saturant based on the weight of the dryweb. The highly resilient nature of the HBA fibers in the web causes theweb exiting the nip of the squeeze rollers to regain approximately thesame thickness as that entering the nip, and thus allow retention of thelow density nature of the web. This saturated product is then driedwithout compression in a suitable apparatus such as an oven at atemperature below 127° C., the melting point of the Celbond® sheathpolymer. This heating step dries the web and crosslinks the Airfiex 120polymer without affecting the integrity of the web which might bereduced by re-melting the Celbond® sheath polymer. The low-compressiondrying is required to avoid crosslinking the binder while the web is ina densified configuration. The final product is a strong, flexible,resilient, abrasion-resistant web suitable for packaging delicate orabrasive articles.

EXAMPLE V

Lightweight (50-100 g) thermobonded HBA pads, either as-is or withsubsequent binder treatment, are suitable for use as a “cushion layer”in a diaper or similar absorbent constructions, providing a means torapidly wick moisture away from the top surface of the construct to alower, relatively heavy, absorbent core. The thermobonded pad providesresistance to flow back to the upper surface because of relatively highthickness of the cushion layer.

Having illustrated and described the principles of the present inventionin a preferred embodiment and variations thereof, it should be apparentto those skilled in the art that the invention can be modified inarrangement and detail without departing from such principles. We claimall modifications coming within the spirit and scope of the followingclaims.

What is claimed is:
 1. A method for producing crosslinked cellulosefibers comprising the steps of: applying a crosslinking substance to amat of cellulose fibers at a fiber treatment zone; conveying the matthrough the fiber treatment zone and directly and immediately into afiberizer without drying and curing the crosslinking substance, thefiberizer having a fiberizer inlet, wherein the fiberizer providessufficient hammering force to separate the cellulose fibers of the matinto a fiber output of substantially unbroken individual cellulosefibers having a nit level of no more than about three; separating thecellulose fibers in the fiberizer by hammering them into the fiberoutput of the substantially unbroken individual cellulose fibers withoutcuring the crosslinking substance; and thereafter drying and then curingthe crosslinking substance to crosslink the individual cellulose fibers,wherein the drying step comprises drying the fibers at a temperature ofabout 200-315° C. so as to flash evaporate water from the fiber outputand form the dried fibers.
 2. A method for producing crosslinkedcellulose fibers, comprising the steps of: applying a crosslinkingsubstance to a mat of cellulose fibers at a fiber treatment zone;conveying the mat through the fiber treatment zone and directly andimmediately into a fiberizer without drying and curing the crosslinkingsubstance, the fiberizer having a fiberizer inlet, wherein the fiberizerprovides sufficient hammering force to separate the cellulose fibers ofthe mat into a fiber output of substantially unbroken individualcellulose fibers having a nit level of no more than about three;separating the cellulose fibers in the fiberizer by hammering them intooutput of the substantially unbroken individual cellulose fibers withoutcuring the crosslinking substance; and thereafter drying and then curingthe crosslinking substance to crosslink the individual cellulose fibers,wherein the fibers are cured at a temperature of about 140-180° C.
 3. Amethod for manufacturing crosslinked cellulose fibers and converting thecrosslinked fibers into a web, comprising the steps of: applying acrosslinking substance to a mat of cellulose fibers at a fiber treatmentzone; conveying the mat through the fiber treatment zone and directlyand immediately into a fiberizer without drying and curing thecrosslinking substance, the fiberizer having a fiberizer inlet, whereinthe fiberizer provides sufficient hammering force to separate thecellulose fibers of the mat into a fiber output of substantiallyunbroken individual cellulose fibers having a nit level of no more thanabout there; separating the cellulose fibers in the fiberizer byhammering them into output of the substantially unbroken individualcellulose fibers without curing the crosslinking substance; thereafterdrying and then curing the crosslinking substance to crosslink theindividual cellulose fibers, and adding a thermobonding agent to thedried and cured individual cellulose fibers, to form a mixture that ismade into a web, then heating the web to a sufficient temperature toincrease the wet strength of the web.
 4. The method of claim 3 whereinthe thermobonding agent is a bicomponent fiber comprising a corecomponent and a sheath, wherein the core component has a higher meltingpoint than the sheath component.
 5. The method of claim 4 wherein thesheath component is selected from the group consisting of polyethylene,polypropylene, poly(1-butene), and copolymers and mixtures thereof, andthe core component is selected from the group consisting ofpolypropylene, poly(1,4-cyclohexylene-dimethylene-terephtalate),poly(4-methyl-1-pentene), polyester and copolymers and mixtures of theforegoing.
 6. The method of claim 4 wherein the core is polypropyleneand the sheet is polyethylene, and the heating step comprises heatingthe mixture to about 130-150° C.
 7. The method of claim 3 wherein themixture comprises the cellulose fibers and thermobonding agent in ratiosby weight of 90:5 to 50:50.
 8. The method of claim 7 wherein the mixturecomprises 20-40% by weight of thermobonding agent.
 9. The method ofclaim 3 further comprising the step of applying a binder to the web. 10.The method of claim 9 wherein the step of applying a binder to the webcomprises saturating the web with the binder and impregnating the binderinto the web with pressure on the web.
 11. The method of claim 9 whereinthe binder is selected from the group consisting of carboxymethylcellulose, acrylic, styrene butadiene rubber, polyvinyl chloride,polyvinylidine chloride, polyvinyl alcohol and polyvinyl acetate.
 12. Amethod for manufacturing croslinked fibers and converting thecrosslinked fibers into a pulp sheet comprising the steps of: applying acrosslinking substance to a mat of cellulose fibers at a fiber treatmentzone; conveying the mat through the fiber treatment zone and directlyand immediately into a fiberizer without drying and curing thecrosslinking substance, the fiberizer having a fiberizer inlet, whereinthe fiberizer provides sufficient hammering force to separate thecellulose fibers of the mat into a fiber output of substantiallyunbroken individual cellulose fibers having a nit level of no more thanabout three; separating the cellulose fibers in the fiberizer byhammering them into the fiber output of the substantially unbrokenindividual cellulose fibers without curing the crosslinking substance;and thereafter drying and then curing the crosslinking substance tocrosslink the individual cellulose fibers and adding the crosslinkedfibers to a pulp furnish to form a pulp sheet having increased porosityand impregnability.
 13. The method of claim 12 further comprising thestep of applying a liquid impregnant to the sheet.
 14. The method ofclaim 13 wherein the applying step comprises applying the crosslinkingagent to the sheet.
 15. A method for producing crosslinked cellulosefibers, comprising the steps of: applying a crosslinking substance to amat of cellulose fibers at a fiber treatment zone; conveying the matthrough the fiber treatment zone and directly and immediately into afiberizer without drying and curing the crosslinking substance, thefiberizer having a fiberizer inlet, wherein the fiberizer providessufficient hammering force to separate the cellulose fibers of the matinto a fiber output of substantially unbroken individual cellulosefibers having a nit level of no more than about three; separating thecellulose fibers in the fiberizer by hammering them into the fiberoutput of the substantially unbroken individual cellulose fibers withoutcuring the crosslinking substance; thereafter drying and then curing thecrosslinking substance to crosslink the individual cellulose fibers, andholding the fibers in a retaining station for a selected period of timeafter drying them.
 16. The method of claim 15 wherein the holding stepcomprises holding the cellulose fibers in a retaining station aftercuring them.
 17. A method for producing crosslinked cellulose fibers,comprising the steps of: applying a crosslinking substance to a mat ofcellulose fibers at a fiber treatment zone; conveying the mat throughthe fiber treatment zone and directly and immediately into a fiberizerwithout drying and curing the crosslinking substance, the fiberizerhaving a fiberizer inlet, wherein the fiberizer provides sufficienthammering force to separate the cellulose fibers of the mat into a fiberoutput of substantially unbroken individual cellulose fibers having anit level of no more than about three; separating the cellulose fibersin the fiberizer by hammering them into the fiber output of thesubstantially unbroken individual cellulose fibers without curing thecrosslinking substance; and thereafter drying and then curing thecrosslinking substance to crosslink the individual cellulose fibers,wherein the drying step comprises flash drying the separated fibers thenintroducing them into an expansion chamber.
 18. The method of claim 17wherein the drying step comprises introducing the cellulose fibers intothe expansion chamber through a venturi.
 19. The method of claim 17wherein the flash drying step comprises conveying the individualcellulose fibers are conveyed from the fiberizer to the expansionchamber by a conduit that decreases in diameter downstream from thefiberizer to increase a flow velocity of the cellulose fibers beforethey enter the expansion chamber.
 20. A method for producing crosslinkedcellulose fibers, comprising the steps of: applying a crosslinkingsubstance to a mat of cellulose fibers at a fiber treatment zone;conveying the mat through the fiber treatment zone and directly andimmediately into a fiberizer without drying and curing the crosslinkingsubstance, the fiberizer having a fiberizer inlet, wherein the fiberizerprovides sufficient hammering force to separate the cellulose fibers ofthe mat into a fiber output of substantially unbroken individualcellulose fibers having a nit level of no more than about three;separating the cellulose fibers in the fiberizer by hammering them intothe fiber output of the substantially unbroken individual cellulosefibers without curing the crosslinking substance; and thereafter dryingand then curing the crosslinking substance to crosslink the individualcellulose fibers, wherein the crosslinking agent is selected from thegroup consisting of methylolated urea, methylolated cyclic ureas, loweralkyl substituted cyclic ureas, dihydroxy cyclic ureas, lower alkylsubstituted dihydroxy cyclic ureas, methylolated dihydroxy cyclic ureas,and mixtures thereof.
 21. A method for producing crosslinked cellulosefibers, comprising the steps of: applying a crosslinking substance to amat of cellulose fibers at a fiber treatment zone; conveying the matthrough the fiber treatment zone and directly and immediately into afiberizer without drying and curing the crosslinking substance, thefiberizer having a fiberizer inlet, wherein the fiberizer providessufficient hammering force to separate the cellulose fibers of the matinto a fiber output of substantially unbroken individual cellulosefibers having a nit level of no more than about three; separating thecellulose fibers in the fiberizer by hammering them into the fiberoutput of the substantially unbroken individual cellulose fibers withoutcuring the crosslinking substance; and thereafter drying and then curingthe crosslinking substance to crosslink the individual cellulose fibers,wherein the pH of the cellulosic fibers remains above about 2 after thecrosslinking agent is applied to the mat.
 22. The method of claim 21wherein the pH is within the range of 2-4.
 23. The method of claim 22wherein the pH is within the range of 3-4.
 24. A method for producingcrosslinked cellulose fibers, comprising the steps of: applying acrosslinking substance to a mat of cellulose fibers at a fiber treatmentzone; conveying the mat through the fiber treatment zone and directlyand immediately into a fiberizer without drying and curing thecrosslinking substance, the fiberizer having a fiberizer inlet, whereinthe fiberizer provides sufficient hammering force to separate thecellulose fibers of the mat into a fiber output of substantiallyunbroken individual cellulose fibers having a nit level of no more thanabout three; separating the cellulose fibers in the fiberizer byhammering them into the fiber output of the substantially unbrokenindividual cellulose fibers without curing the crosslinking substance;and thereafter drying and then curing the crosslinking substance tocrosslink the individual cellulose fibers, wherein the separating stepcomprises separating the cellulose fibers in a hammermill comprising: ahousing; an elongated rotor within the housing and having a longitudinalaxis of rotation, the rotor including a plurality of hammers havingdistal end surfaces sweeping out an effective rotor surface uponrotation of the rotor about the axis of rotation, the distal endsurfaces of the individual hammers upon such rotation sweeping separatecylindrical paths with gaps between the paths, the gaps between thepaths not exceeding about one-quarter of an inch; a rotator that rotatesthe rotor about the axis of rotation to thereby rotate the hammers toprovide the effective rotor surface; and the hammermill including atleast one inlet through which a fiber mat is delivered to the effectiverotor surface for fiberization by the rotating hammers, the housingdefining an outlet located at an intermediate position corresponding toan intermediate portion of the effective rotor surface between the endsof the rotor, the outlet extending substantially the entire length ofthe housing.